Blender Cloud Generator Edges
That's due to the step size being too large. Volume rendering works by a process called ray marching. In short, the volume is sampled at various distances and summed to get the total light coming from that bit of volume. The step size controls the distance between these samples.

In general, the bigger the step size, the fewer places the volume needs to be sampled, so the faster the render time. However, in return, you get these slicing artifacts.



To fix it, try lowering the step size in Properties > Render > Geometry > volume sampling (2.7x), or Properties > Render > Volumes (2.8).